It really depends upon what your needs are. VoIP is a relatively mature technology, and usually delivers very good audio quality. If you are just looking to save on international calls, then almost any company will do. If you are a call center or business, and need things like IVR, Queues, Follow-me, Fax or other technology's, then your needs are different.
At SysNetx, we provide VoIP infrastructure and we are very familiar with VoIP and VoIP services. We are happy to help you make a decision, so visit us at www.sysnetx.com & navigate to our contact page. We are happy to hear from you.